1. Field of the Invention
The present invention relates to the modification of battery grids of the type used in lead-acid storage batteries, and more particularly, it relates to a modification of the surface finish of the battery grids of a lead-acid storage battery to improve paste adhesion and the service life of the battery.
2. Description of the Related Art
Lead-acid storage batteries typically comprise several cell elements which are encased in separate compartments of a container containing sulfuric acid electrolyte. Each cell element includes at least one positive plate, at least one negative plate, and a porous separator positioned between each positive and negative plate. The positive and negative plates each comprise a lead or lead alloy grid that supports an electrochemically active material. The active material is a lead based material (i.e., PbO, PbO2, Pb or PbSO4 at different charge/discharge stages of the battery) that is pasted onto the grid. The grids provide an electrical contact between the positive and negative active materials which serves to conduct current.
Lead-acid battery manufacturing technologies and materials have improved dramatically in the last few decades. One early revolution was the use of battery grid materials that produce a “maintenance-free” battery. Because pure lead is too soft for the manufacturing processes used to form battery grids, various alloying elements have been added to lead over the years to produce battery grids of sufficient strength to withstand battery manufacturing processes. For example, antimony was added to lead as lead-antimony alloys were found to be capable of being formed into battery grids at acceptable commercial rates by way of gravity casting techniques. However, it was discovered that when a lead-antimony alloy is used in battery grids, water loss occurs because of gassing. Therefore, batteries having lead-antimony grids required periodic maintenance, i.e., the addition of water to the battery. In order to lower the gassing rate of batteries, lead-calcium battery grids were developed. Batteries using lead-calcium alloy grids have low gassing rates, and therefore, do not require the addition of water. As a result, the use of lead-calcium alloy battery grids has led to the introduction of “maintenance-free” batteries.
Another significant revolution in lead-acid battery manufacturing has been the manufacturing of battery plates in a continuous process, instead of the traditional methods in which battery grids are made using a conventional gravity cast book mold operation and the cast grids are later pasted in a separate step. In a typical continuous battery plate making method, a lead alloy strip is manufactured, either by casting (namely, cast strip) or by casting and rolling (namely, wrought strip), and the strip is subsequently expanded or punched to generate the desired grid pattern in a strip of interconnected battery grids. Typically, lead alloys having a relatively high level of calcium are used in continuous grid making processes as higher calcium levels tend to increase the hardness of the battery grids which is beneficial in punching and expansion processes. Previously prepared active material battery paste (which may be prepared by mixing lead oxide, sulfuric acid, water, and optionally dry additives, such as fiber and expander) is then applied to the strip of interconnected battery grids and the strip is parted into single battery plates. The main advantages of continuous battery plate making are production rate, dimensional control, thinner plates, lower strap rate and lower manufacturing costs. The pasted plates are next typically cured for many hours under elevated temperature and humidity to oxidize free lead (if any) and adjust the crystal structure of the plate. After curing, the plates are assembled into batteries and electrochemically formed by passage of current to convert the lead sulfate or basic lead sulfate(s) to lead dioxide (positive plates) or lead (negative plates). This is referred to as the “formation” process.
It is well known that lead-acid batteries will eventually fail in service through one or more of several failure modes. Among these failure modes is failure due to corrosion of the grid surface. Electrochemical action corrodes the grid surface and reduces the adhesion between the active material and the grid. In most instances, failure of the battery occurs when the grids are no longer able to provide adequate structural support or current flow due to the separation of the active material from the grid. It has been determined that negative lead-acid battery plates made by a continuous plate making method as described above have performed at least as well in service (cycle) life as negative plates made from conventional gravity cast book mold grids. However, positive lead-acid battery plates made by a continuous plate making method underperform in service (cycle) life as compared to gravity cast book mold grids, especially in the high temperature environment under the hood of today's more compact cars. In particular, lead-acid batteries having positive plates made by a continuous plate making method from lead-calcium alloys have proven to be relatively short-lived as determined by the SAE J240B Life Cycle Test (at 40° C. and particularly at 75° C.) owing to corrosion of the grid surface which forms an electrically resistive layer between the active material and the grid and seemingly reduces the adhesion between the active material and the grid over the course of the test. Lead-calcium grid batteries are particularly susceptible to early failure for the high temperature (75° C.) J240 test, and are short-lived compared to similar batteries made with lead-antimony grids.
Therefore, there have been efforts to improve the service life of a lead-acid battery having continuously manufactured plates, particularly by increasing the adhesion of positive grids to the active paste material. For example, a method for extending the cycle life of a lead-acid storage battery is disclosed in U.S. Pat. No. 5,858,575. In this method, a continuous length of unexpanded strip or a continuous length of preexpanded grid strip, each of which is formed from a lead-calcium alloy, is coated with a layer of a tin, lead-antimony, lead-silver or lead-tin alloy by hot dipping in a melt of the alloy. The layer of metal on the surface of the grid promotes better adhesion of the active material paste to the grid.
Another similar method is described in U.S. Pat. No. 4,906,540 which discloses a method wherein a layer of a lead-tin-antimony alloy is roll-bonded to a strip formed of a lead-calcium alloy. The strip is then expanded into a continuous length of grids. It is stated that the surface layer of the lead-tin-antimony alloy enables the battery active material to be retained for a long period of time. The increased adhesion of the paste to the grid serves to improve the cycle life of the battery.
Yet another similar method is described in Japanese Patent Publication No. 10-284085 which discloses a method wherein a coating of a lead-antimony-selenium alloy is fused to a lead-calcium-tin alloy strip and the strip is thereafter punched and/or expanded to form battery grids. The grids formed by this process are believed to increase battery life.
Still another similar method is described in U.S. Pat. No. 4,761,356 which discloses a method wherein a lead-calcium alloy strip is coated with a lead-tin alloy by dipping, spray coating or plating, and the coated alloy strip is thereafter punched or expanded to form a continuous strip of battery grids. The use of a process wherein the lead-calcium strip is punched or expanded after coating with a lead-tin alloy produces a grid with the lead-calcium alloy exposed at areas where grid material is punched out of the strip. The alloy coating is reported to improve recovery after over-discharge.
The formation efficiency of lead-acid batteries also depends to a great extent on the positive plate, in particular, to the extent of conversion of lead monoxide (PbO) to lead dioxide (PbO2) in the active positive material. The high electrical potential required for formation appears to be related to the transformation of non-conductive paste materials to PbO2. A low formation efficiency of positive plates requires a high formation charge. Inefficient charging also leads to deficiencies in the resulting batteries assembled with such plates. Typically, the initial capacity (performance) of the battery is low if the battery is not completely formed, requiring additional cycling to reach specific performance values. It is well known that by increasing the adhesion between the paste mixture and the grid, formation efficiency can be improved. Among other things, the increased adhesion between the grid and the paste provides for improved interfacial contact between the grid and paste thereby improving current flow between the grid and paste.
Thus, it can be seen that the adhesion between a battery grid and battery active material may affect, among other things, battery formation processes and battery service life. Accordingly, various methods, such as those mentioned above, have been proposed to improve the adhesion between a battery grid and battery active material, and thereby improve battery service life.
However, all of the aforementioned methods have certain disadvantages that limit the ability of these methods to attain maximum effectiveness in improving battery service life. For instance, the methods disclosed in U.S. Pat. Nos. 4,906,540 and 4,761,356 and Japanese Patent Publication No. 10-284085 all form a battery grid by applying an alloy coating to a strip and thereafter punching or expanding the strip to form battery grids. As a result, the alloy coating will not be present on the grid wire surfaces that face the openings that are formed in the strip when the strip is punched or slit and expanded. Therefore, the beneficial effects of the alloy coating on paste adhesion and service life will be necessarily limited as the entire surface of the grid wires will not be coated. In addition, the coating methods disclosed in U.S. Pat. No. 5,858,575 are used with expanded metal grids (as shown in FIG. 1 of U.S. Pat. No. 5,858,575) which are known to have inferior charge/discharge efficiency as compared to stamped grids, such as that shown in U.S. Pat. No. 5,989,749. The decreased charge/discharge efficiency of the expanded grids also limits the service (cycle) life of a battery.
Therefore, there continues to be a need in the battery manufacturing field for even more effective methods for improving the service life of a battery. More particularly, there is a need for a method that can more greatly increase the adherence of active material to a battery grid produced by a continuous process.